With Date::Manip:

$ cat 714749.pl
use strict;
use warnings;

use Date::Manip;

# YYYYDOY -> YYYYMMDD

while (<DATA>) {
        m/(\d{4})(\d{3})/;
        my ($y,$m,$d,$h,$mn,$s) = Date_NthDayOfYear($1,$2);
        printf("%04d-%02d-%02d\n", $y, $m, $d);
}

__DATA__
2008001
2008002
2008003
2008004
2008005
2008360
2008361
2008362
2008363
2008364
2008365
2008366
[download]

$ perl 714749.pl
2008-01-01
2008-01-02
2008-01-03
2008-01-04
2008-01-05
2008-12-25
2008-12-26
2008-12-27
2008-12-28
2008-12-29
2008-12-30
2008-12-31
[download]